Method for determining susceptibility of urea to caking on an accelerated basis

ABSTRACT

UREA IN ANY OF A VARIETY OF FORMS, EITHER WITH OR WITHOUT ANTICAKING ADDITIVES, IS PLACED IN A PERFORATED MOLD AND SUBJECTED TO AN ENVIRONMENTAL CYCLE COMPRISING A FIRST HALF CYCLE OF A HIGH TEMPERATURE-LOW HUMIDITY ENVIRONMENT AND A SECOND HALF-CYCLE OF A LOW TEMPERATURE-HIGH HUMIDITY ENVIRONMENT TO THEREBY SIMULATE A TYPICAL DIURAL CYCLE. THE UREA IS MAINTAINED AT A PRESSURE OF 2 TO 6.5 P.S.I.G., DURING THE CYCLE AND FROM 2-1/2 TO 20-1/2 CYCLES ARE REQUIRED TO PRODUCE USEFUL RESULTS. THE METHOD PRODUCES ACCURATE AND USEFUL QUANTITATIVE DATA WITH TEST DURACTIONS AS SHORT AS 0.3 DAYS ALTHOUGH A TEST PERIOD OF 2-4 DAYS IS PREFERABLE. THE METHOD FINDS UTILITY BY PROVIDING A QUICK EVALUATION OF THE EFFICACY OF VARIOUS UREA ANTICAKING ADDITIVES.

United States Patent 3,501,942 3/1970 Fitzgerald [72] Inventors Richard Lapham Gilbert. Jr. 73/l5.4

Princeton Primary Examiner-James J. Gill H pp No sg ggg Knapp ardnne' Assistant Examiner-Herbert Goldstein Filed p 1969 Attorney- Lawrence W. Flynn [45] Patented June 28,1971 [73] Assignec American Cyanamid Company. Stanford.

Conn.

[54] METHOD FOR DETERMINING SUSCEPTIBILITY OF UREA TO CAKING ON AN ACCELERATED BASIS ABSTRACT: Urea in any of a variety of forms, either with or 4 Claims 4 Drawing Figs without antlcakmg addit ves, is placed in a perforated mold and sub ected to an environmental cycle comprising a first [52] [1.5. CI 73/154, ha|f cycle of a high temPeratureJow humidity environment 73/ 81 and a second half-cycle of a low temperature-high humidity [5i] Int. Cl G01 25/00, environmem to hereby Simulate a typical diurnal cycle The 3/48 urea is maintained at a pressure of 2 to 6.5 p.s.i.g., during the [50] Field of Search 73/l5.4, 78, cycle and from 23km zo %cycles are required to produce 84 ful results. The method produces accurate and useful quan- References Cited titative data with test durations as short as 0.3 days although a test period of 2-4 days is preferable. The method finds utility UNITED STATES PATENTS by providing a quick evaluation of the efficacy of various urea 3,481,187 l2/l969 Passmore 73/81 anticaking additives.

CRUSH STRENGTH, LB.

HARD/V585 0F UREA CAKES V5. NUMBER OF TEST CYCLES N0 CO/VD/ T/O/VE R 3% KAOL//V\ /0 l2 l4 l6 /8 NUMBER OF TEST CYCLES SHEET 1 BF 2 PATENTED JUN28 l97| 30 TEMPERA TURE (DRY BUL B) INVENTORS RICHARD 1.. G/LBERT JR. BYPAUL m KNAPP N0 CO/VD/ T/O/VER I APPLIED PRESSURE (PS/6) jwdh w (U: 72

AGE/VT EFFECT OF PRESSURE PATENT ED'JUN28 I971 SHEU 2 OF 2 R S Ev s m M m C m M A C N E T 0 M S m 0 F8 N OV S 0 M R N E D B R M A U H N m M m .5

m u Ik QEMWK m IWDWG NUMBER OF TEST CYCLES PER CENT LUMPS //v BAG TEST CAK/NG INDEX //V ACCELERATED TEST CAK/NG lNDEX AGE/VT METHOD FOR DETERMINING SLISCEPTIBILIT" OF UREA TO CAKING ON AN ACCELERATED BASIS BACKGROUND OF THE INVENTION The problem of fertilizer caking has existed since the beginning of the manufacture of synthetic fertilizers. Many fertilizer components, or their reaction products with other components are hygroscopic. lt is known (Silverberg, J. et al., J. Ag. Food Chem. 6 442. [1958]) that caking is promoted by movement of moisture which leads to,formation of crystal bridges between particles. With the advent of high analysis fertilizers. particularly ammonium nitrate, urea and mixtures containing these materials as components, problems of caking have multiplied.

In the investigation of factors responsible for caking, and in development of conditioners to reduce caking, it is necessary to have a caking test for evaluation.

The traditional caking test is the stacked bag test, in which the test sample, in a standard fertilizer bag, is placed at the bottom ofa stack of bagged goods to bags high. In some tests the bagged goods are replaced by an equivalent weight. The extent of caking of the sample is evaluated after 1 to 3 months. Evaluation is usually qualitative, by inspection, or semiquantitative, by screening the material and weighing lumps, or by counting the number of times the bag must be dropped to break up lumps.

This type of test is a real" test, in that it shows what would happen in an actual storage situation. Although it is too cumbersome and time consuming to be used in screening potential conditioners, it is the ultimate test with which any accelerated test must be correlated. 1

Modified bag storage tests using smaller amounts of material have been reported (Bridger, G. L., et al., Proc. Fertilizer Round Table p. 94[ 1966], lannicelli. 1. Proc. Fertilizer Round Table, p. 89[l966], Pierce, .1. 5., Abstracts l52nd National Meeting of American Chemical Society. New York, N.Y., Sept. 1966 p. J20, Rosenblatt. T. M. and Geissler, P. R., Abstracts l52nd National Meeting of American Chemical Society New York, N.Y. Sept. 1966 p. J21). Although these tests reduce the amount of material needed, they do not materially reduce the length of time required for a test.

Numerous accelerated caking tests have been developed in the past years. A pressure bomb test (Adams, J. R. and Ross, W. H., Ind. Eng. Chem. 33 l20[l94) was applied to several types of fertilizer materials in an extensive investigation of the variables influencing test results. This test has been used (Hardesty, .l. O. and Kumagai. R., Agricultural Chem, Feb. 38, Mar. 55[1952], Kumagai, R. and I-lardesty, .l. 0., J. Ag. Food Chem. 4 l32[1956]) in an investigation of the effectiveness of conditioners on granularmixed goods. In this test the hardness of the cake formed in the bomb, by mechanical pressure, was measured by crushing it in a hydraulic press.

In another test applied to mixedgoods (Whynes, A. L. and Dee, T. P., Jour. Sci. Food Agriculture 8 577[l957]), a cake was formed by air pressure on a sample sealed in a rubber I 401[1949]. Varma. S.. et al..J. Sci. Ind. Res. 188 118[1959].

Wilson, J. R., et al., Agricultural Chemicals Sept. p. 42(1962], and lannicelli [1966] supra). Whetstone and Varma allowed ammonium nitrate to absorb water, and then to dry, and measured the hardness of the'cake by crushing or with a penetrometer. Wilson and lannicelli caused caking of ammonium nitrate by repeated cycling through the 32 C. transition point, while maintaining the material under pressure. Both measured the. hardness of the cake by crushing with a hydraulic press.

Other tests of a general nature have been described (Mischel, P. 8., Farm Chemicals Sept.. p. 43[1967], and Parks]. R. and GranokJ. Farm Chemicals Oct..p. 5 l[l967]). No test. however. has been designed specifically for evaluating caking of urea, and none of the tests described above is completely satisfactory for such use. A major disadvantage of the Adams and Ross pressure bomb test is the problem of sealing the joints of the bomb to prevent passage of water vapor. Furthermore, this test cakes urea only when the water content is relatively high.

Through use of a rubber sleeve to form cakes, the Whynes and Dee test tends to give cakes of variable dimensions, a disadvantage when the strength of the cake is to be measured by crushing.

Tests developed for ammonium nitrate, which depend on repeated c-ycling through the 32 C. transition point to produce caking do not work with urea, which has no such transitions.

In 1966 Geissler and Rosenblatt discussed simulated storage tests on various fertilizers at the Sept. Meeting of the American Chemical Society (New York, N.Y.) and indicated that urea is the only fertilizer about which major reservations are harbored with regard to simulated storage results. At the present time, applicants are unaware of any known method which is entirely suitable for evaluating the caking characteristics of urea on an accelerated or simulated basis.

The method of this invention provides an accelerated cak' ing test, specifically for use with urea, the results of which correlate with stacked bag tests. The method depends upon cycling the test sample, under pressure in a perforated mold, through high temperature-low humidity and low temperaturehigh humidity conditions, simulating day and night summer atmospheres.

SUMMARY OF THE INVENTION This invention relates to a method for determining the susceptibility to caking of urea or urea containing anticaking agents on an accelerated basis. This method, moreover, provides quantitative results which correlate with actual bag testing in fertilizer plants.

The term urea as used hereafter means urea per se and urea which has been treated with one or more additives with the purpose of reducing the undesirable caking tendencies of urea during storage. The term urea is meant to include all of the physical forms of urea commonly utilized in the fertilizer industry such as, for example, urea crystals, microprills, prills, fertilizer grade prills, granulated urea and such. The precise descriptions of such forms of urea are well known to those skilled in the art and do not bear repeating herein.

In accordance with the method of this invention the susceptibility of urea to caking can be accurately and quantitatively determined by a. placing a sample of urea in a perforated mold and subjecting it to an environment cycle consisting of a first half cycle of i. from about 2 to about'8 hours at a dry bulb temperature of about F. to 120 F., a pressure of about 2 to 6.5 pounds per square inch gauge (p.s.i.g.) and relative humidity which is below the critical humidity of urea at the temperature selected but in excess of 10 percent followed by a second half cycle of ii. from about 1 to about'5 hours at a dry bulb temperature of about 60 to 80 F., a pressure of about 2 to 6.5 p.s.i.g. and a relative humidity of 70 percent to percent and then b. measuring the crush strength of the urea as treated in (i) and (ii).

The above cycle must be repeated at least 2 /2 times and may be repeated as many as 20% times with satisfactory results.

The first half cycle (i) simulates the high temperature-low humidity conditions ordinarily existing during daytime in the summer. The second half cycle (ii) simulates the low temperature-high'humidity conditionsexisting at nighttime during the summer. By combining both half cycles a diurnal cycle is simulated. a factor of substantial significance with regard to the accuracy and reliability of themethod.

Moreover, it has been found that if pressures in excess of 6.5 p.s.i,g. are used, the results become inconsistent and contradictory when compared with the results of bag testing resulting in a unreliable test procedure.

In carrying out the first half cycle (i) it becomes necessary to know the critical humidity of urea at the selected temperature. FIG. 1 presents a plot of the critical humidity of urea as a function of temperature and can be used for this purpose.

The method of this invention finds utility by providing an accelerated quantitative method for determining the efficacy of various urea anticaking additives. The only heretofore satisfactory method of evaluation in the case of urea was the bag tests of at least 1 month duration and preferably 3 months duration. As discussed hereinabove bag tests were at most semiquantitative in nature. The method of this invention provides useful quantitative results with test periods ranging from only 0.3 to l 1 days in duration, with test periods of 2 to 4 days being preferred. The crush strength measurement is an indication of the degree of caking and is readily converted to a cake index by simply dividing the crush strength of an untreated urea control sample by the crush strength of the sam-' ple. The cake index can then be used as a quantitative basis for comparing the efficacy of various anticaking agents added to the urea.

The test is similarly useful in arriving at the optimum amounts of anticaking agent required or at the proper order of addition when more than one agent is to be used.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plot of the critical humidity of urea as a function of dry bulb temperature.

FIG. 2 depicts the effect of pressure applied during accelerated testing upon crush strength of the urea cake.

FIG. 3 depicts the effect of the number of environmental cycles to which the urea is subjected in the accelerated test upon crush strength of the urea cake.

FIG. 4 is a plot of the caking index as obtained from the accelerated test method of this invention vs. the percent lumps in an actual inplant bag test and serves to illustrate the validity of the accelerated test method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Test Molds The mold used may be of any suitable geometric configuration and may exhibit wide variation in the type and distribution of perforations. Thus it may be square, rectangular, cylindrical, spherical or such. It is desirable that the perforations be such as to permit the urea to be contained in the mold in a substantially rigid and unchanging geometric configuration in order to obtain a urea sample of substantially uniform dimensions from test to test and thereby reduce errors in the measurement of the crush strength of the sample. For the same reason, the perforated mold should possess sufficient rigidity to resist deformation when pressure is applied to the urea sample contained therein.

It is important to maintain constant dimensions of the sample cake from test to test in order to avoid inconsistencies in the crush strength measurement.

The crush strength is the maximum force required to push a pointed spike into the sample until there is no further increase in pressure. The crush strength measurement is a standardized and well-known test procedure and a variety of commercial equipment is available for performing it.

Environmental Conditions In a series of preferred embodiments of the invention, the

first halfcycle (i) conditions are:

ll l0" F. 50-55 percent relative humidity 3-4 p.s.i.g., and 4-6 hours and the second half cycle conditions are:

6870 F. 78-82 percent relative humidity 3-4 p.s.i.g.

LII

1-3 hours with the number of total cycles ranging from 8% to IS'A.

A specific set of conditions which has produced excellent test results is as follows:

first half cycle:

104 F. 53 percent relative humidity 3 p.s.i.g. 4 hours second half cycle:

68 F. 8l percent relative humidity 3 p.s.i.g. 2 hours for a total of 1 HS cycles.

It is desirable to always begin and end the environmental conditioning of the urea with the high temperature-low humidity half cycle, i.e. the first half cycle (i). When this procedure is followed the total number of cycles used will always be expressed in terms of an integer and the fraction, A. For example if one begins with the first half cycle (i) and carries out I I complete cycles, the test will conclude at the end of the second half cycle of the l lth cycle. Since it is desirable to conclude the test with the first half cycle (i) conditions, the test will be carried an extra half cycle concluding at the end of the first half cycle (i) thus resulting in l 1% cycles.

Equipment and Test Procedures A general outline of the preferred equipment and procedures employed is provided below:

Equipment 1. Molds in which urea cakes were fonned were split cylinders, 2 inches in diameter and 2 inches high, made of 16 gauge perforated metal with one-sixteenth inch perforations on oneeighth inch centers (triangular pitch). On either end of the cylinder, to retain the urea, was placed a disc of 0.004 inch polyethylene film and a disc of one-sixteenth inch aluminum sheet 1% inches in diameter.

2. The test rack, in which the molds were placed for application of pressure, comprised an inverted "T" made of threeeighths inch aluminum plate. The test molds rested on the base formed by the cross bar of the inverted "T," while on either side of the vertical perpendicular plate were bolted three air cylinders. (Alkon Products, Wayne, NJ. Series D Model 24, 6 inches stroke). Each test rack could therefore accept six molds. Low pressure air was supplied to the cylinders through a Moore Nullmatic regulator (Model 41-30). Pressure was measured by a test gauge.

3. The test racks were set inside an American Instrument Climate-Lab, Model 45500. Cams for the cycle timer of this instrument were cut to give the desired time-temperature-relative humidity.

4. Hardness of the urea cakes was measured with a Dillon Multi Low Range Tester, with a compression cage. The upper platen of the cage was drilled and tapped to accept a %20 NC stainless steel machine screw, the end of which was turned to a 60 cone.

5. Solid conditioners were applied to urea experimentally with a Patterson-Kelly laboratory model Twin-shell blender. Liquids were applied by spraying with a DeVilbiss atomizer onto a rolling bed of urea prills, in a pill-coater or similar device.

Procedures l. Formation of Cakes The bottom of the test mold was covered with a piece of paper, held on with masking tape. An aluminum disc and then a polyethylene disc were put in the bottom of the mold, 50 grams of urea prills was added, and finally another polyethylene and an aluminum disc were placed on top of the urea. The polyethylene facilitates removal of the aluminum discs at the conclusion of the test. The assembly was placed in the test rack directly under the air cylinder. The test rack was placed in the Climate-Lab, and the desired air pressure applied under the desired conditions of temperature and humidi- Twelve samples were tested simultaneously The position of the samples in the test rack was chosen at random. In general each set of samples was tested three times (further replication did not improve precision) and the average value of cake hardness was calculated. In a series of tests the average value of cake hardness was calculated. In a series of tests the average standard deviation was found to be :l020 percent,

satisfactory for the purposes intended.

2. Measurement ofCake Hardness A 74-20 stainless steel machine screw. with a 60 percent conical end, was installed in the compression cage of the tester, to bear upon the top of the cake.

The test cake, still in its mold, (but with aluminum and polyethylene discs removed) was placed in the cage and the force needed to rupture the cake was measured. The cake ruptured downward, from the point of the spike, in roughly a 60 cone. Most of the prill-prill bonds within this cone of material were broken. Utility The method of this invention is useful as a means of predicting the efficacy of various urea anticaking agents in a very short time as compared to the 1 month or 3 month bag test which had previously been used.

Before an accelerated method can be accepted, it must be demonstrated that it produces results which are in accord with actual results i.e. as indicated by bag tests. For example, it is known that kaolin clay is a widely used urea anticaking agent. Thus a comparison of results from a suitable accelerated test on a kaolin treated urea and untreated urea should indicate a higher degree of caking in the untreated urea than in the treated urea. We have found that in order to obtain such consistency in our procedure, it is imperative that the pressure be kept below about 6.5 p.s.i.g. The reason for this is clearly shown in FIG. 2 wherein the effect of varying pressure upon urea crush strength is presented for an untreated urea sample and urea treated with 3 percent kaolin subjected for 18 hours to a temperature of 130 F. and 55 percent R. H. followed by a 2 hour period at 130 F. and 25 percent R. H. The data of FIG. 2 show that as pressure exceeds 6.5 p.s.i.g. the test is no longer indicative of the actual effect observed since the treated urea has a higher crush strength greater than that of the untreated urea.

FIG. 3 presents data illustrating the effect of the number of cycles upon the test results where the environmental conditions consist ofa first cycle (i) at 104 F., 53 percent R. H. and 3 p.s.i.g. for 4 hours and a second cycle (ii) at 68 F., 81 percent R. H., 3 p.s.i.g. for 2 hours. The data show that a comparison of the crush strength of the untreated and treated urea corresponds over a larger number of cycles to that actually observed in bag tests. To obtain accurate results, it is desirable to use at least 2% cycles in order to be in the range where significant differences in crush strength begin to appear between the treated and untreated urea and where the crush strength values are of sufficient magnitude to permit their measurement without undue error.

To validate the test stacked-bag tests were performed at two producing plants: one at Niagara Falls, Canada, and the second near New Orleans, La. Individual batches of experimentallyconditioned urea microprills were made at each site, using freshly produced prills. Bags were stacked high, with the test bags at the bottom. The condition of the samples was evaluated by plant personnel after 1 month and after 3 months by carefully slitting the bags, screening the contents on a onehalf inch screen, and reporting the percentage of the material caked. Attempts were also made to evaluate the hardness of the lumped urea.

At the time the batches were made, a portion of each was evaluated by the accelerated test. Samples of unconditioned urea were tested at the same time. Cake hardness for the unconditioned urea from the two plants-was not the same, numerically. It is possible that this variation is caused by different particle-size distribution. or .by difference in initial water or biuret content. To put conditioner tests on the same basis, results of the test were calculated as a caking index," said index computed by dividing the hardness of the uncondi- TABLE I.-(O.\'IPARISON OF ACCELERATED TEST WITH BAG STORAGE TEST Bag test Accelerated test Percent lumps (3 months) Caking index Plant A Plant B Plant A Plant B Conditioner system:

All lumps reported medium hardness by both plants, except for System l-Plant A.

Mirroring, as they do, the differences in preparation at two widely separated plants, by different personnel, on different substrates; storage under widely different climatic conditions; and evaluation by different operators, the data provide adequate verification of the test.

The data of FIG. 4 indicate that changes in the cake index between values of 1.0 and about 6.0 have profound effects on the percentageof lumps found in actual practice (i.e. the percent lumps varies from 100 percent TO 27 percent). However, as the index increases above 6.0 there seems to be little effect on the percent lumps (ie the percent lumps varies from 27 percent to only l8 percent). Thus the data indicate that in using the method of this invention to evaluate the efficacy of new urea anticaking agents, any agent having a cake index of 6.0 or greater would be most satisfactory. Those having an index less than 6.0 would offer varying degrees of efficacy. Most presently used commercial urea anticaking agents produce cake indices between 1.0 and 6.0.

The cake index is an extremely useful figure because it permits a quantitative comparison of the efficacy of various urea anticaking agents. Untreated urea will always have a cake index of 1.0 while urea treated with effective anticaking agents will show indices in excess of 1.0 with the magnitude of the difference from 1.0 being an indication of their efficacy. Ureas containing uneffective additives on the other hand will show an index of 1.0 or less.

The following examples are provided to further illustrate the invention.

EXAMPLE I I invention in evaluating the efficacy of various anticaking addil tives for urea microprills on an accelerated basis.

A variety of anticaking agents were l applied to urea microprills (-20 60 mesh containing about 0.2 to 0.3 percent water) from a given lot. An untreated sample from the same lot was selected as a control.

The control sample and the various tre ted samples were exposed to a first half cycle of 4 hours at 104 F. (dry bulb), 3 p.s.i.g., and 53 percent R. H. and a second half cycle of 2 hours at 68 F. (dry bulb), 3 p.s.i.g. and 81 percent R. H. for a total of H5 cycles. This corresponds to total test time of only 70 hours or about 3 days. When the environmental cycles were completed, the crush strength of the urea cake in the mold was measured and the cake index computed therefrom. The equipment and procedures employed ,were as described hereinafter in the section devoted to this top'ic.

Results are shown below in Table II. It should be noted that the test procedure indicates that many of the additives do improve the anticaking characteristics of th'e urea. However, some have the opposite effect and produce a urea having an even greater tendency to cake than the untreated urea.

The additives shown in Table ll are known urea anticaking agents. The cake index values of Table ll confirm for the most part what is known about the efficacy of these particular additives from other sources such as bag tests. This confirmation indicates the usefulness and reliability of the method of this in- 5 vention. t

The data of Table ll also indicate that the method is useful in determining the optimum amounts of each additive when one or more additives is employed.

EXAMPLE 2 This example illustrates the usefulness of the method of this invention in evaluating the efficacy of various anticaking additives for urea crystals on an accelerated basis.

The procedures of Example 1 were followed in all respects except that the urea microprills were replaced by urea crystals. Results are shown below in Table Ill. The same observations made with reference to the data of Table II in Example 1 also apply to the data of Table III.

TABLE lll.---A('CELERATED CARING TEST ON CONDITIONED CRYSTAL I'REA Average crush Percent strength (Taking Additive Type Producer additive pounds index Nonc- (c-ontrol) 76. 7 1.0 I{1045-13. S l. 35. 4 2. 2

Sicaric acid 0.1 R-l0-15-43'l Stai'cl 2. 3T. 5 2.0 Stearic acid 0. 1 R1045-43-1 Starch. 2. 5 30. 6 1. 9 BUTT-113 .00.. 2.0 42.1 1. 8 Rl045431 do 2.0 43.1 1.8 Stcaric acid 0. 1 R97T113 Starch. 2. 5 46 1 1 7 Stcaric acid. 0.1 Dryfio. tearch- 2. 5 51. O 1. 5 Purity S- 0.... 2.0 59. 0 1.5 Hydroseal 3 do 2. 0 64. 8 1. 2 Dryfio and 1%} Ca. stcarate ..do... 1.0 65.9 1.2 Mistrou ZSC Diatornaceous 2.0 69. 7 1.1 Mistron vapor Diatomaceous 2.0 87. 9 0. 9

TABLE H.ACCELERATED TEST RESULTS OF -UREA MICROPRILLS CONDITIONED WITH MISCELLANEOUS MATERIALS Average crush Percent strength, Caking Additive additive lbs. index None (control) 69. 5 1; 0 Ca stcarate. v 0. 2o 6. 3 11. 0 %all oil may aclg. I 56. 8 1. 2

a oi iu -yaci v Kaolin 2. 5 l 2 SD 3171- I 63.6 1.1 SD 3171 s g l 0 5 50. 1 1. 4

1T -o l 555;.

I Kaolin 2. 5 i Kaolin l 3. 0 -13. 4 1. 6 Georgia kaolin (Andersonv1lle). 3.0 44. 2 1. 6 Calcium bentonite 3.0 85. 8 0. 8 Diatornaceous earth 3. 0 87. 9 0. 8 Benpink ball clay 3. 0 98. 5 0. 8 Term K cla v 3.0 98.0 0.7 Sodium hentorute. 3.0 100 0. 7 Miss M and D 3.0 100 0. 7 Dark \\'ad 6 3. 0 100 0. 7 Pakistan llmll cla\ 03.18 100 0. 7 M nera oi. KQOMIL l 0 2 i 81.0 0. 9 Minera oi a fii l l 0 53 1, 55 5 1. 3 l more 01 I Kaolin 2. 5 l 4 3 Molojel 0.20 66. 9 l. 0 Fght vgii agol 70. 5 1. 0

sopar g g 035., 100 0. 7 55 sopar Eao1in\ n 03g 97 0. 1

opar 1 Kaolin 2. 5 i 08 Cake curb 11 0. 10 100 0. 7 Lauryl alcohol 0. 10 100 0. 7 Anneen 11-11 0. 10 100 0. 7

l. Barnet Clay, agricultural kaolin, 74 percent minus 1 EXAMPLE 3 This example illustrates the usefulness of the method of this invention in providing an accelerated quantitative basis for evaluating the effects of changes in the order of addition of anticaking additives upon the efficacy of the additives.

The procedures of Example 2 were followed in all respects. Results are shown below in Table IV.

TABLE IV.EFFECT OF ORDER OF CONDITIONER AP- PLICATION ON CRYSTAL UREA Crush Secondary strength, Primary conditioner conditioner lb.

0.5% fatty acid derivative 2.5% Kaolin. 11.8 2.5% Kaolin 0.5% fatty acid 22. 1

derivative. 0.5% fatty acid derivative (added to urea at the same time 35. 2 2.5% Kaolin 0.5% fatty acid derivative (Kaolin treated with fatty acid derivative). 42. 9 2.5% Kaolin EXAMPLE 4 The procedures of Example 1 were followed in all respects except that the environmental cycle consisted of a first half cycle of 8 hours at F. (dry bulb), 2 p.s.i.g., and 10 percent R. H. and a second half cycle of 1 hour at 60 F. (dry bulb), 2 p.s.i.g., and 70 percent R. H. Six such tests were run for each additive studied wherein the total number of cycles in each test was 2.5, 8.5, 11.5, 14.5, 17.5 and 20.5. The three additives studied were kaolin, calcium stearate, and tall oil fatty acid. The results were plotted as shown in FIG. 3 with the resulting plots in all cases being substantially as shown in FIG. 3. Satisfactory correlation of the results of the accelerated tests with inplant tests was also obtained.

EXAMPLE 5 The procedures of Example I were followed in all respects except that the environmental cycle consisted of a first half cycle of 2 hours at 120 F. (dry bulb), 6.0 p.s.i.g., and 60 percent R. H. and a second half cycle of 5 hours at 80 F. (dry bulb), 6.0 p.s.i.g., and 100 percent R. H. Six such tests were run for each additive studied wherein the total number of cycles in each test was 2.5, 8.5, U5, 14.5, 17.5 and 20.5 and the additives were those used in Example 4. As in Example 4, the resulting data plotted substantially as shown in FIG. 3. Satisfactory correlation of the results of the accelerated tests with inplant tests was also obtained.

We claim:

1. A method for determining, on an accelerated basis, the

susceptibility to caking of urea or a urea containing an anticaking agent which comprises the steps of: v

a. subjecting said urea contained in a perforated mold to an environmental cycle consisting ofa first half cycle of:

i. from about 2 to about 8 hours at a dry bulb temperature of about 90 to 120 F.. relative humidity below the critical humidity of urea 'at the temperature selected but greater than l percent. and a pressure of about 2 to 6.5 p.s.i.g.. followed by a second halfcycle of ii. from about 1 to about hours at a dry bulb temperature of about 60 to 80 F, relative humidity of 70 to lOO percent and a pressure of about 2 to 6.5 p.s.i.g. for a total number of cycles ranging from 2 to ending at the end ofa first halfcycle, and then b. measuring the crush strength of said treated urea.

2. The method of claim 1 wherein the first halfcycle conditions are:

100 to l l0 F. (dry bulb) 50 to 55 percent relative humidity 3 to 4 p.s.i.g.

4 to 6 hours and the second half cycle conditions are:

68 to 70 F. (dry bulb) 78 to 82 percent relative humidity 3 to 4 p.s.i.g.

l to 3 hours 3. The method of claim 2 wherein the number of cycles is from about Sto 15%.

4. The process of claim 3 wherein the first half cycle conditions are about:

104 F. (dry bulb) 53 percent relative humidity 3 p.s.i.g.

4 hours and the second half cycle conditions are about:

68 F. (dry bulb) 81 percent relative humidity 3 p.s.i.g.

2 hours and wherein the number of cycles is 1 1%. 

